KINETIC TRAPS IN POLYMER ADSORPTION .2. POLYSTYRENE DISPLACED BY POLYISOPRENE AT 50-DEGREES-C

Abstract

We analyze adsorption-desorption kinetics at significantly higher temperatures than in the accompanying study (Schneider, H. M.; Granick, S.; Smith, S. preceding paper in this issue) so as to contrast enthalpic, detachment-limited desorption with the diffusion-limited regime. The experiments involve measurements of protiopolystyrene (PS) and deuterio-cis-polyisoprene (PI) adsorbed sequentially onto oxidized silicon from a dilute solution in carbon tetrachloride at 48.5-degrees-C. The time dependence of the displacement of PS by PI is analyzed as it depends on the PS molecular weight (M(PS)). In contrast to behavior at low temperature, if M(PS) is sufficiently low (<10(5)), the PS desorption is a simple exponential in elapsed time, as expected for desorption that is rate-limited by segmental surface detachment. Larger M(PS) chains, however, even at this elevated temperature, display kinetics that is strongly nonexponential in time. When fit to a stretched exponential as suggested by a simple kinetic model, the power of time is beta congruent-to 0.6, near the beta = 0.5 limit expected for diffusion-limited desorption. In contrast to behavior at low temperature, a prominent overshoot of PI surface excess occurred for adsorption onto PS of low molecular weight. This overshoot, which is also seen for PI adsorption onto a bare surface, is tentatively attributed to initially rapid unequilibrated adsorption, followed by conformational rearrangement of adsorbed PI.